Problem with extract_compute using Fortran wrapper

I think I fixed it---the problem arose because the compute arrays for
per-atom data aren't allocated contiguously in memory (i.e., they're
pointers to an array of pointers), so C_F_pointer was returning garbage.
The solution is to call the Cdoublestar_to_2darray function I wrote
instead of the C_F_pointer intrinsic.

Thanks for catching this---I had not actually tested any array-valued
computes yet. I need to double-check the other procedures as well to
make sure I haven't made the same mistake elsewhere, then I'll send the
changes.

-Karl